U.S. Pat. No. 3,580,055 to Richard B. White discloses a test apparatus for measuring the tightness of tanks, particularly underground fuel storage tanks. That apparatus has concentric suction and discharge tubes which extend down through the fill pipe into the underground tank and a pump at ground level connected to the upper ends of those tubes to provide a continuous recirculation of the fuel out of and back into the tank. A temperature sensor is provided on the lower end of the suction tube. At ground level the apparatus has a calibrated standpipe for controlling the hydrostatic pressure of the fluid in the tank. An adapter sleeve extends part way down into the fill pipe around the outside of the suction tube. A radially expandable annular seal of short vertical extent on the outside of the adapter sleeve is operated by a handle at ground level to provide a seal between the adapter sleeve and the inside of the fill pipe. Extensive practical experience has shown that this seal arrangement is not entirely satisfactory in many installations, and had to be supplemented with a similar seal of larger diameter and other fittings.